Use a header to show a title in the canvas
Pep8 fixes
Remove lessons

The lessons content was included in the Help activity
Move options to a palette
Improve selection screen style
White background in the build poll screen
Improve style on edt build. Add visual feedback on validation
Style improvements on vote screen
Show object chooser with image preview
Better header in the poll screen
Remove code to implement default poll, because is not used
Use colors in the graphic bars

Use the same logic (and code) from the Finance activity.
The file colors.py is copied from that activity.
Use json to save/restore to the journal

Previous version used pickle. This version can open objects
with the old format to keep compatibility.
Fix initialization of options palette

The palette was created before the journal object was read,
then the initial values were always wrong.
This patch also rename variables to make clear what is private,
and use th eright naming style.
If no polls saved, open the build poll screen
When select a poll from the list reset preview state
Improve vote screen, based on Dan proposal
Apply the config "show preview" without need restart the screen
Remove inconsistent variable current_view
Apply 'Use image' config without need restart screen